It is widely known in the art that the processing of crude oil and its various fractions has led to damage to piping and other associated equipment due to naphthenic acid corrosion. Generally speaking, naphthenic acid corrosion occurs when the crude being processed has a neutralization number or total acid number (TAN), expressed as the milligrams of potassium hydroxide required to neutralize the acids in a one gram sample, above 0.2. It is also known that naphthenic acid corrosion is more severe when the naphthenic acid-containing hydrocarbon is at a temperature between about 200.degree. C. and 400.degree. C. (approximately 400.degree. F.-750.degree. F.), and also when fluid velocities are high or liquid impinges on process surfaces e.g. in transfer lines, return bends and restricted flow areas. Additional background on the problem of naphthenic acid corrosion in oil refineries can be found in Gutzeit, Materials Performance, pp. 24-35, October, 1977; Piehl, NACE Corrosion 87 Meeting, Paper No. 196, Mar. 9-13, 1987; and Scattergood et al., NACE Corrosion 87 Meeting, Paper No. 197, Mar. 9-13, 1987.
Various approaches to controlling naphthenic acid corrosion have included neutralization and/or removal of naphthenic acids from the crude being processed; blending low acid number oils with corrosive high acid number oils to reduce the overall neutralization number; and the use of relatively expensive corrosion-resistant alloys in the construction of the piping and associated equipment. These attempts are generally disadvantageous in that they require additional processing and/or add substantial costs to treatment of the crude oil. Alternatively, various amine and amide based corrosion inhibitors are commercially available, but these are generally ineffective in the high temperature environment of naphthenic acid corrosion. U.S. Pat. No. 4,941,994 to Zetlmeisl et al. discloses a naphthenic acid corrosion inhibitor comprising a dialkyl or trialkyl phosphite in combination with an optional thiazoline. However, phosphorous compounds are known to impair the function of various catalysts used to treat crude oil, e.g. in fixed-bed hydrotreaters and hydrocracking units. Crude oil processors are often in a quandary since if the phosphite stabilizer is not used, then iron can accumulate in the hydrocarbon up to 10 to 20 ppm and impair the catalyst.
Naphthenic acid corrosion is readily distinguished from conventional rusting and other types of corrosion. Naphthenic acid corrosion occurs in a non-aqueous environment and produces a characteristic grooving of the metal in contact with the corrosive stream that seems to distinguish it from high temperature sulfur attack. Rusting, on the other hand, is generally caused by water in contact with the steel surface in an oxidizing environment. Thus, various sulfonates, primarily metal and amine salts of aromatic sulfonic acids, e.g., dodecylbenzene and dinonylnaphthalene, have been used to treat metal surfaces to protect them from corrosion caused by contact with moisture and air by forming a rust preventive coating on the metal surface. In general, the coating includes a polar layer tightly bound to the metal surface by chemisorption, and a non-polar or barrier layer, generally a hydrophobic material, which blocks corrosive molecules such as oxygen and water from getting to the metal surface. See, Gustavsen et al., NACE Corrosion 89 meeting, paper no. 449, Apr. 17-21, 1989. However, conventional rust preventives such as dodecylbenzene sulfonate and dinonylnaphthalene sulfonate are generally ineffective for inhibiting naphthenic acid corrosion.
Accordingly, it would be very desirable to have available a naphthenic acid corrosion inhibitor which is both effective at naphthenic acid corrosion temperatures and suitable for use in hydrocarbons being processed with a catalyst.